1. Field of the Invention
The present invention relates to an information recording apparatus in which a photosensitive body is exposed to a beam from a semiconductor laser, driven in response to a recording signal, to record desired information.
2. Related Background Art
The shortest oscillation wavelengths of existing semiconductor lasers fall within the range of 760 to 810 nm.
An electrophotographic photosensitive body must be sensitized with infrared rays. In this case, the sensitivity of the photosensitive body varies within the semiconductor laser wavelength range. In addition, wavelengths of semiconductor lasers vary depending on semiconductor chip temperatures.
Laser wavelengths vary in response to temperature changes at a rate of 0.2 to 0.3 nm/.degree.C. For example, if the laser chip temperature is changed from standard room temperature (i.e., 25.degree. C.) to 35.degree. C., the wavelength is increased by 2 to 3 nm. However, if the laser chip temperature is changed from standard room temperature to 5.degree. C., the laser wavelength is shortened by 4 to 6 nm.
An electrophotographic photosensitive body used together with a semiconductor laser has sensitivity in the semiconductor laser oscillation wavelength range of 660 to 830 nm. The sensitivity greatly varies in the range of 760 to 810 nm (the sensitivity is decreased for a longer wavelength), as shown in FIG. 6. In FIG. 6, the wavelength is plotted along the abscissa and the relative sensitivity of the photosensitive body is plotted along the ordinate.
As is apparent from FIG. 6, the sensitivity of the photosensitive body and therefore, the surface potential of the portion of the photosensitive body irradiated with a laser beam changes greatly in accordance with changes in the wavelength of the semiconductor laser. A conventional technique has been proposed to set the laser wavelength to be constant so as to stabilize the quality of the recorded image.
In a conventional information recording apparatus using a semiconductor laser, a Peltier element which can be heated and cooled is used to maintain the laser chip temperature and its ambient temperature to be constantly 20.degree. to 25.degree. C. However, in order to maintain the laser chip temperature at 20.degree. to 25.degree. C., a voltage with positive and negative polarities and a large current are required for the Peltier element. Therefore, a bulky, large-capacity power source is required.
The Peltier element as a whole is bulky since it requires a heat sink, and the element itself is not always stable. Further, when room temperature is abruptly increased from a low to a high temperature, the area near the laser chip is maintained at 20.degree. to 25.degree. C., and condensation may form on the glass window of the semiconductor laser unit, scattering the output beam, thereby resulting in blurring or disturbance of the image.
The temperature characteristics of the laser wavelengths in different semiconductor lasers vary. The wavelengths of laser outputs at an identical temperature vary in different semiconductor lasers. Therefore, if a semiconductor laser mounted in an information recording apparatus is replaced with an equivalent one, image quality is often degraded.